(a) Field of the Invention PA1 (b) Description of the Prior Art
The present invention relates to a phenolic resin composition. More specifically, the present invention pertains to a phenolic resin composition for use in making resin-coated sand particles which do not give out a bad smell and do not generate any soot and smoke during formation of a mold by the usual shell molding method and which have good disintegration properties after casting.
In general, phenolic resins have conventionally been used as resins for coating sand particles employed in the shell molding method. This is because the cured phenolic resins have excellent strength and rigidity at ordinary and high temperatures, and a relatively low and stable price.
Resin-coated sand particles are charged in a preheated mold and the charged sand particles are heated through heat conduction so that the resin on the surface of the sand particles becomes molten together and cured to give a mold. Most of these molds thus formed are cores, which are incorporated into principal molds produced according to other processes so that molten metal can be casted in the mold. The resin is heat-decomposed upon pouring the molten metal so that a mold can be obtained through treating processes such as cooling and knock-out processes. The term "core" herein used means a part used for forming a cavity in a casting.
The resins for coating sand particles which are used for producing a mold according to the shell molding method should satisfy the following requirements: 1) they must permit the formation of a mold having high strength; 2) they must have good disintegration properties after casting; and 3) they must not produce a bad smell and must not generate much soot and smoke during forming the sand particles into a mold and casting a mold.
There have been known various methods for preparing resins capable of providing a mold having high strength such as a method in which a resin having a low molecular weight is employed and a method which makes use of phenol resins modified with alkyl phenols. However, if a resin having a low molecular weight is used, the fusion temperature thereof is reduced and it is liable to cause blocking. Moreover, conventional phenol novolak resins having low molecular weights which are in general available in the market have high contents of phenol and/or binuclear phenolic components which result in production of a bad smell and generation of large amounts of soot and smoke. On the other hand, the method which makes use of alkylphenols suffers from a problem of low hardening rate. Improving the flow properties of resin-coated sand (RCS) particles and improving the packing density of the resin in a mold are very important for the enhancement of the strength of the resulting mold. The flow properties can be improved by reduction of the molecular weight of a resin used or by addition of a lubricant to a resin. However, the amount of such a lubricant to be added is limited to a small amount and if the amount thereof exceeds 5% by weight, the strength of the resulting mold is conversely impaired.
In the shell molding method, molten metal surrounds a core during pouring the molten metal in the mold and, therefore, the atmosphere thereof is completely free of oxygen. For this reason, the phenolic resin is carbonized and thus easily disintegrated after cooling. In this case, however, the mold often holds high strength and thus the problem of low disintegration properties arises. In particular, aluminum has recently been used for the production of cars to lighten the weight thereof and to thus reduce fuel expenses. However, the foregoing phenomenon is problematic for the core of a low temperature-casted mold and the mold as such cannot be used because of poor disintegration properties. For this reason, the disintegration properties of the core produced at a low molten metal-pouring temperature have presently been improved by subjecting the core to sand-baking at a temperature ranging from 400.degree. to 500.degree. C. for several hours after casting, but this results in an increase in its cost. Accordingly, there has been desired, in the field of castings, the development of a resin for a shell mold having excellent disintegration properties after casting. Many attempts have been made for the improvement of the disintegration properties of resins for shell molding. For example, a small amount of an agent for accelerating disintegration has, in fact, been used in most of such techniques.
Japanese Examined Patent Publication (hereinafter referred to as "J.P. KOKOKU") Nos. Sho 62-32015 and Sho 61-41655 disclose resin compositions having good disintegration properties which make use of the residues obtained through purification or cleavage of bisphenol A. These patents utilize a mixture of the residues obtained through purification or cleavage of bisphenol A with a phenol novolak resin which is generally available in the market, or a resin obtained through a condensation reaction of the residues obtained through purification and cleavage of bisphenol A with phenol and formaldehyde. Therefore, these resin compositions often have high contents of binuclear phenols originating from the phenol novolak resin and optionally comprise unreacted phenol.
Moreover, a gas producing a bad smell and containing soot and smoke is generated when a mold is produced through the use of resin-coated sand particles. This causes a social problem in that the gas impairs the environmental condition in and around mold-production factories. It has been believed that the production of a bad smell and the generation of soot and smoke are caused by the presence of formalin, ammonia, phenol, binuclear phenols and low molecular weight decomposition products originating from other additives. When the resins having excellent disintegration properties disclosed in the foregoing patent are used, they result in the sever production of a bad smell and generate large amounts of soot and smoke.
Japanese Un-examined Patent Publication (hereunder referred to as "J.P. KOKAI") No. Hei 4-93311 discloses that a resin hardly producing a bad smell can be obtained by reduction of the contents of mono- and bi-nuclear components of a novolak type phenol resins. However, the resulting resin has a molecular weight higher than the starting phenol novolak resin and poor flow properties since the resin is obtained by simply removing the mono- and bi-nuclear components from the starting phenol novolak resin generally available in the market. If the resin used has a high molecular weight, the resin holds its strength even after the sand-baking process and this results in the formation of a mold inferior in the disintegration properties.
As has been discussed above, there has not yet been proposed any resin for shell molding which maintains satisfactory disintegration properties and can solve the problems of producing a bad smell and generation of soot and smoke, during formation of a mold.